Efforts in the design of integrated circuits for radio frequency (RF) communication systems generally focus on improving performance, reducing cost or a combination thereof. One area of increasing interest relates to conversion of signals, such as from analog-to-digital or digital-to-analog. Both types of conversion have benefited from the development and use of delta-sigma modulation.
Delta-sigma modulation is a technique used to generate a coarse estimate of a signal using a small number of quantization levels and a very high sampling rate. Limiting a signal to a finite number of levels introduces “quantization” noise into the system. Oversampling and the use of an integrator feedback-loop in delta-sigma modulation are effective in shifting quantization noise to out-of-band frequencies. The noise shifting properties enable efficient use of subsequent filtering stages to remove noise and produce a more precise representation of the input.
The use of delta-sigma modulation in a digital-to-analog conversion can produce an analog signal having a high dynamic range, but only for limited ranges of frequency. Outside of a narrow frequency band having a high dynamic range, the delta-sigma modulation produces significant quantization noise. In effect, the delta-sigma modulator trades the majority of its spectral range for a few regions of high dynamic range operation. The out-of-band quantization noise produced by the delta-sigma modulation requires an analog filter to attenuate all frequencies outside of the narrow low-noise bands.
While delta-sigma modulators have been useful in digital-to-analog conversion applications dealing with a narrow band of frequencies, frequency hopping and multi-frequency applications have required a wide-band, convention, digital-to-analog converter to achieve necessary spectral coverage. The performance of existing digital-to-analog converters using delta-sigma modulators, referred to hereinafter as a delta-sigma DACs, simply degrades too quickly to be useful outside of its narrow spectral band. Delta-sigma DACs possess a number of desirable characteristics, such as a high dynamic range and the ability to directly produce radio frequency signals, that would make them useful for these applications. The present invention enhances the utility of delta-sigma DACs, enabling them to support a wider range of frequencies and in turn support multi-band and multi-standard transmitters.